An adaptive antenna array is an array of antennas connected to a communications receiver and operates by combining the signals received by the antennas so as to optimize in an adaptive fashion the receive characteristics of the array. By weighting and then summing the multiple antenna signals, the adaptive antenna array can adapt its angular response, sometimes called the array pattern, while it operates, in response to changes in the propagation environment. While operating, the adaptive antenna attempts to maximize the reception of the signal received from a desired transmitting device, while simultaneously minimizing the effects of all other unwanted interfering signals and noise. In a communication system, the interference suppression capability of an adaptive antenna array offers the potential to reduce co-channel interference, improve coverage quality, and increase overall system capacity.
A problem associated with employing an adaptive antenna in a wireless communication system lies in the design of a combining algorithm and device that has adequate ability to track changes in the signal environment. Adaptive antennas that operate in a fast-fading multipath environment should adapt to the rapidly varying channel as well as to any changes in the nature of the desired and interfering signals. The ability of the antenna combining algorithm to track a faded channel will directly impact its overall performance. Algorithms which cannot track channel variations suffer significant degradation in performance as measured by the bit-error rate (BER) or signal-to-interference-plus-noise ratio (SINR).
In a communication system, a practice known in the art is for a transmitting device to transmit a plurality of pilot symbols and a plurality of data symbols to a receiving device. The data symbols contain the information (such as voice, data, or multimedia information) that is communicated to the receiver by the transmitter. The receiving device has prior knowledge of the nature of the transmitted pilot symbols and can use the received pilot symbols to perform such tasks as carrier recovery, channel estimation, and other related tasks that are known in the art for maintaining a high-quality communication link. In order to exploit the allocated spectrum most efficiently, it would be advantageous to minimize the percentage of the transmitted symbols that are pilot symbols.
In a commonly used adaptive antenna technique, the array combining weights are computed from a known sequence of pilot symbols, and these weights are then applied to subsequently received data symbols. In this technique, the portion of the allocated time-frequency spectrum in which pilot symbols are transmitted by the desired transmitter is known as the training interval. The interval in which the weights are applied to the data symbols is known as the application interval. For best performance, these adaptive antenna techniques require the channel and signal characteristics to be relatively constant over both the training interval and the application interval. If the channel changes significantly within either interval, the performance will suffer. Furthermore, if the channel is effectively constant within both intervals but is different from interval to interval, then the performance will also suffer. Therefore, to effectively suppress interference, an antenna signal combiner must have the ability to track variations in the channel. For best efficiency, the number of pilot symbols required by the combiner should be kept to a minimum.
The design of adaptive antenna systems that exploit pilot symbols becomes one of finding the best balance between performance and efficiency. Increasing the number of pilot symbols will provide better performance in a static channel, but will reduce the system efficiency as defined by the ratio of information data symbols to pilot symbols. However, using a larger number of pilot symbols will increase the likelihood that the channel will vary over the training and application intervals, which is a situation that will result in degraded performance. On the other hand, using fewer pilot symbols will improve the likelihood of a static channel, but using too few pilot symbols can also degrade performance in a static channel. As a result, there is a need for a method and device that utilize adaptive antenna techniques that satisfy the contradicting goals of maximizing efficiency while simultaneously being able to track variations in the channel.